Observations on Cacti in Cultivation 163 



four hundred, while many had fallen and other buds were ap- 

 pearing. The walls of the ovaries and the involueral bracts 

 were thicker than the same parts on Sentinel Hill plants. 



The Sahuarito plant is quite well protected from wind, 

 and while open to the sun all about, is subjected to strong in- 

 solation in the afternoon. Under its influence, coupled with 

 plenty of rich moisture, photosynthesis is probably at its height, 

 while the warm and temporary dry period at the date of observa- 

 tion, perhaps acting especially through the medium of the 

 sandy soil, undoubtedly brought on the unusual wealth of flowers. 



A CASE OF STIMULUS AND RESPONSE IN OPUNTIA. 



In the enclosure of the Santa Cruz hotel at Tucson, Arizona, 

 there is a planted specimen of Opuntia with flat, spineless joints. 

 On June 3, 1909, and for several days preceding, the numerous 

 stamens of the large light-yellow flowers then in bloom were seen 

 to exhibit interesting movements when touched. As the flowers 

 unfold, the anthers radiate outward until all the stamens stand 

 at a greater of less angle from the vertical. A very slight touch 

 with a pencil will cause them to travel inward, at the same time 

 an occasional strong whiff of wind will shake them without effect. 

 If pushed outward, there was a strong reactive movement in- 

 ward until the stamens stood nearly erect next the pistil. If 

 pushed inward, little movement took place, and this was apt to 

 occur in any direction. If the stamens were depressed to the 

 right on the near side, there was little or no motion beyond the 

 resumption of their first position; if, however, they were bent to 

 the left, they not only regained their original position, but kept 

 moving varying distances in the opposite direction, the anthers 

 sometimes traveling the distance of a centimeter before coming 

 to a halt. If the bending stress was applied in the same direction 

 all about the flower, the reflex response manifested itself by the 

 rotation about the pistil of the entire body of anthers, coming 

 to a stop only when the filaments had assumed a strongly lean- 

 ing position opposite to that given by the stimulating object. 

 If the same stress was applied the other way around, the fila- 

 ments on coming to rest stood nearly erect, as before. 



The same flowers were found on succeeding days with their 

 stamens in the original position, and the application of the same 



